In general, metal foils such as aluminum foils are used, in lithium-ion batteries, as collectors (supports) to which positive-electrode materials and negative-electrode materials are made to adhere. However, metal foils have a two-dimensional structure and hence are inferior in terms of carrying of active materials and packing density of active materials to porous bodies. Specifically, metal foils cannot hold active materials in a manner in which metal foils contain active materials. Accordingly, metal foils cannot suppress expansion or contraction of active materials and hence the amount of active materials held on metal foils is made small to ensure a life for a certain period. In addition, the distance between collectors and active materials is long and hence active materials away from collectors are less likely to be used. Thus, the capacity density becomes low. Metal foils are used in the form of a porous body such as a punched metal body, a screen, or an expanded metal body. However, these also substantially have two-dimensional structures and hence a considerable increase in the capacity density cannot be expected.
To achieve a higher output, a higher capacity, a longer life, or the like, many collectors that are, for example, three-dimensional porous bodies such as foam or nonwoven fabric have been proposed (refer to Patent Literatures 1 to 4).
For example, Patent Literature 1 discloses, as a positive-electrode collector, a three-dimensional network porous body whose surface is composed of aluminum, an alloy, or stainless steel. Patent Literature 2 discloses that an electrode mixture in which a porous polymer is uniformly distributed between active-material layers and on the surface of the active material is integrated with a collector that is a three-dimensional porous body composed of a metal such as aluminum, copper, zinc, or iron, a conductive polymer such as polypyrrole or polyaniline, or a mixture of the foregoing, to thereby form an electrode.
Patent Literature 3 discloses an electrode in which an electrode active-material thin-film layer is formed on a porous collector composed of an element of aluminum, tantalum, niobium, titanium, hafnium, zirconium, zinc, tungsten, bismuth, or antimony, an alloy of the foregoing, or a stainless-steel alloy.
Patent Literature 4 discloses that an aluminum foam, a nickel foam, or the like is used as a positive-electrode collector.
In general, to provide secondary batteries having a higher output and a higher capacity, there has been a demand for collectors that are three-dimensional structures, which are more porous than two-dimensional structures. In particular, since positive-electrode collectors are susceptible to oxidation by electrolytes under a high charging-discharging voltage, positive-electrode collectors having sufficiently high oxidation resistance and electrolytic resistance have also been demanded.
Three-dimensional metal structures having a high porosity (hereafter, referred to as “porous metal bodies”) are generally produced by making a porous non-conductive resin body be electrically conductive, electroplating this porous resin body with a predetermined amount of a metal, and, if necessary, removing the remaining inner resin portion by incineration. For example, Patent Literature 5 states that a porous metal body is produced by plating the skeleton surface of a polyurethane foam with nickel and then removing the polyurethane foam. Patent Literature 6 describes a fuel-cell collector produced by forming a metal-plated layer containing fine particles composed of a fluorine-based resin having high water repellency, on the surface of a porous nickel-material base, and performing press-forming.
However, positive-electrode collectors that have oxidation resistance and electrolytic resistance, have a high porosity, and are suitable for industrial production, are not provided for lithium nonaqueous-electrolyte secondary batteries for the following reasons.
Specifically, in general, to produce a collector having a high porosity such as a porous nickel body serving as a typical example, the surface of a porous organic resin is plated and, if necessary, the organic resin is removed by incineration. However, porous nickel bodies are susceptible to oxidation in lithium nonaqueous-electrolyte secondary batteries and dissolved in electrolytic solutions. Accordingly, such batteries are not able to be sufficiently charged after charging and discharging are performed for a long period of time.
On the other hand, in order to perform plating with aluminum, which currently serves as a main material of positive-electrode collectors, molten salt at a very high temperature needs to be used. Accordingly, organic-resin bodies cannot be plated and it is difficult to plate organic-resin surfaces. Thus, porous aluminum collectors are not currently provided.
Stainless steel is also widely used as a material of positive-electrode collectors. However, for the same reason as for aluminum, it is also difficult to provide collectors having a high porosity by plating organic-resin surfaces with stainless steel.
Note that the following method is provided: a porous stainless-steel body is produced by applying stainless-steel powder to a porous organic-resin body and sintering the applied powder.
However, stainless-steel powder is very expensive. In addition, a porous organic-resin body to which the powder adheres is removed by incineration and the resultant body has a poor strength and is not usable, which is problematic.
Accordingly, there is a demand for a collector that has oxidation resistance and electrolytic resistance, has a high porosity, and is suitable for industrial production; and a positive electrode including such a collector.